COBOL code:
PROCESS-DATA.
ADD 1 TO COUNTER.
DISPLAY COUNTER.
IF COUNTER while (counter I wrote COBOL code for a few years at a job when I was a teenager. What makes legacy COBOL code difficult IMO is it can sometimes be very hard to maintain a mental execution state when examining the code, for several reasons:
1. all variables are global, aka, WORKING-STORAGE. You list all the variables used in the program and they are accessible to the entire program.
2. programs are divided into paragraphs. Control normally flows sequentially top to bottom through paragraphs, one executing after another. Except that the PERFORM statement can drastically alter this normal flow control, and you can't tell by looking at a paragraph how it will be executed. To do that, you have to look at all PERFORM statements that mention this paragraph or any paragraph physically before it, because in COBOL you can say PERFORM PARA1 THROUGH PARA27. If PARA13 is physically between PARA1 and PARA27, it's potentially going to get executed.
3. In true legacy COBOL, before structured COBOL was a thing (circa 1985), the main control flow statement in addition to PERFORM was GOTO. Lots of flag setting, and lots of GOTOs. So in the previous example, you can't tell if PARA13 is going to get executed because any prior statement might be a GOTO PARA14, skipping execution of PARA13. But even worse, you are still under the influence of the PERFORM THRU, so after PARA27 is executed, control returns to the statement following the PERFORM THRU, wherever that was. But if you GOTO PARA27, without being under a PERFORM THRU, then PARA27 is executed followed by the next sequential paragraph. Trying to figure this out statically by looking at the program can be very difficult, especially considering PERFORMs that are nested at runtime but may not be anywhere near each other in a code listing.